The present invention is directed to improved polybutadiene compositions suitable for molded golf ball core construction. The improved polybutadiene compositions contain specific naturally occurring, essentially non-reinforcing, crystalline silicas which have the effect of increasing the resilience and/or compression properties of the resultant molded cores. In addition, the present invention is also directed to golf balls produced utilizing the improved core compositions.
Two of the principal properties involved in the performance of golf balls are resilience and hardness. Resilience is determined by the coefficient of restitution (C.O.R.), the constant "e", which is the ratio of the relative velocity of two elastic spheres after direct impact to that before impact, or more generally, the ratio of the outgoing velocity to incoming velocity of a rebounding ball. As a result, the coefficient of restitution (i.e. "e") can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision. Hardness is determined as the deformation (i.e. compression) of the ball under various load conditions applied across the ball's diameter (i.e. the lower the compression value, the harder the material).
Resilience (C.O.R.), along with additional factors such as club head speed, angle of trajectory, and ball configuration (i.e. dimple pattern), generally determines the distance a ball will travel when hit. Since club head speed and the angle of trajectory are not factors easily controllable, particularly by golf ball manufacturers, the factors of concern among manufacturers are the coefficient of restitution (C.O.R.) and the surface configuration of the ball.
In this regard, the coefficient of restitution of a golf ball is generally measured by propelling a ball at a given speed against a hard surface and measuring the ball's incoming and outgoing velocity electronically. The coefficient of restitution must be carefully controlled in all commercial golf balls in order for the ball to be within the specifications regulated by the United States Golfers Association (U.S.G.A.). Along this line, the U.S.G.A. standards indicate that a "regulation" ball cannot have an initial velocity (i.e. the speed off the club) exceeding 255 feet per second (250 feet per second with a 2% tolerance). Since the coefficient of restitution of a ball is related to the ball's initial velocity (i.e. as the C.O.R. of a ball is increased, the ball's initial velocity will also increase), it is highly desirable to produce a ball having a sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of hardness (i.e. impact resistance) to produce enhanced durability.
The coefficient of restitution (C.O.R.)in solid core balls is a function of the composition of the molded core and of the cover. In balls containing a wound core (i.e. balls comprising a liquid or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings.
With the overall desired properties of golf ball construction in mind, filler and/or reinforcement agents may be added to golf ball core compositions in order to increase the compression of the core and/or to increase the core weight so that the finished ball more closely approaches the U.S.G.A. upper weight limit of 1.620 ounces. The filler and/or reinforcement agents are generally inert particulate matter present in finely divided form, for example, less than about 20 mesh, and preferably less than about 100 mesh U.S. standard screen size. Examples of such inert particles normally utilized in golf ball core construction as reinforcing agents include non-crystalline or amorphous silicas such as fumed, precipitated or hydrated silicas (U.S. Pat. No. 3,756,607), fiberglass (U.S. Pat. No. 3,218,075), graphite (U.S. Pat. No. 4,473,229), sulfates such as barytes and calcium sulfate, zinc oxide, carbon black, titania, alumina fibers, cork, cotton flock, leather fiber, plastic fiber, leather flour, plastic flour, asbestos, talc, and metal oxides and carbonates. However, a number of these filler agents, particularly zinc oxide and titania are expensive and while they effectively add weight to the core, they are relatively ineffective (with the exception of zinc oxide which is effective to some degree at low levels) in enhancing the hardness and resilience of the resulting molded cores.
Along the same line, the Reiter patent (U.S. Pat. No. 4,688,801) demonstrates the use of several fibrous fillers which are also ineffective to some degree for improving resilience. The so-called "active hydrogen containing" fillers disclosed therein (i.e. wood-flock, cotton flock (cellulose-cotton), nylon flock (nylon 6--6), solka-flock (cellulose-wood), cork dust (suberin/cellulose), rayon, etc.) are reported to increase the fracture load and compression (hardness) while also significantly reducing resilience (% rebound). The same patent also demonstrates the effectiveness of inorganic fillers (barytes, zinc oxide, and Franklin Fibers) in decreasing the rebound (i.e. resilience) percentages and, depending on the filler, either increasing or decreasing compression. However, the fact that Reiter did not achieve positive results from the use of many of these fillers including the fumed silicas is attributed to differences in the formulations used, the nature of the filler additives, and/or differences in the compounding/curing methodologies utilized.
The present invention relates to the addition of specific non-amorphorous or crystalline (preferably microcrystalline) silica particles to polybutadiene core compositions in order to produce solid golf ball cores and/or centers exhibiting enhanced resilience and/or compression characteristics. It has been found that the addition of specific naturally occurring crystalline silica particles to the polybutadiene compositions results in an increase of the coefficient of restitution (of from about 0.10% to about 1.00%) over that exhibited by similar cores prepared with a number of inorganic fillers, such as the current standard calcium carbonate fillers.
The findings of the present invention are surprising and unexpected due to the great number of fillers, including reinforcing silicas and/or non-crystalline or amorphous silicas, which produce poorer performance characteristics with respect to resilience (C.O.R.) than the calcium carbonate fillers currently utilized in the trade. As a result, the solid golf ball cores and/or centers of the present invention can be used to produce unitary or wound golf balls approaching the U.S.G.A. initial velocity limit without having to place great reliance on the C.O.R. produced by the specific type of cover and/or windings utilized in the overall construction.
Accordingly, it is an object of the present invention to provide improved polybutadiene core compositions which, when utilized to formulate golf balls, produce golf balls exhibiting enhanced distance and/or durability due to the composition of the core. An additional object of the invention is to provide cores or centers for the construction of solid or wound golf balls which approach the initial velocity limit set by the U.S.G.A. without placing significant emphasis on the coefficient of restitution produced by the balls' cover and/or windings. Furthermore, since a golf ball produced in accordance with the present invention exhibits an enhanced coefficient of restitution and thus, initial velocity, it is possible to increase the durability of the ball by adding additional durability enhancers without greatly sacrificing the ball's overall resilience. A still further object of the invention is to produce golf ball cores or centers which maintain their roundness and shape upon continued impact by a golf club.
The foregoing has outlined some of the most pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or by modifying the invention within the scope of the disclosure. Accordingly, other objects and a more detailed understanding of the invention may be had by referring to the summary of the invention, the detailed description of the invention and the claims which follow below.